The invention relates to a process for the production of a percarboxylic acid product, e.g. a peracetic acid product and a perpropionic acid product, by feeding hydrogen peroxide, acetic acid and water continuously into an aqueous reaction medium containing hydrogen peroxide, acetic acid, percarboxylic acid and an acid catalyst, wherein hydrogen peroxide and acetic acid react in the presence of an acid catalyst and form percarboxylic acid, and by removing an aqueous percarboxylic acid concentrate continuously from the reaction medium by distillation.
The invention also relates to a percarboxylic acid product of the above type and its use for combating microorganisms.
Below, the concentration percentages used in the specification of the invention indicate percentages by weight. Additionally there are used acronyms AA=acetic acid, PAA=peracetic acid, dPAA=distilled peracetic acid, ePAA=equilibrium mixture.
The conventional process for producing percarboxylic acid, such as peracetic acid, is based on an equilibrium reaction between it and water and between acetic acid and hydrogen peroxide, in a reaction medium:
AA+H2O2⇄PAA+H2O 
The catalyst used in this case is a strong mineral acid, the most common being sulfuric acid. The equilibrium mixture is prepared simply by mixing the components together. The composition of the equilibrium mixture depends, among other things, on the molar ratio of the starting materials.
Stronger and purer aqueous solutions of percarboxylic acid are in general produced by distilling percarboxylic acid from the reaction medium under a reduced pressure and at an elevated temperature. Thereby an aqueous percarboxylic acid concentrate is obtained, which contains percarboxylic acid, water and in general a small amount of carboxylic acid and possibly a very small amount of hydrogen peroxide.
Patent publication EP 0 789 016 A1 discloses a process for the production of peracetic acid. In the process, hydrogen peroxide and acetic acid are allowed to react in an aqueous solution that contains hydrogen peroxide, acetic acid, peracetic acid, sulfuric acid and a stabilizer (phosphonic acid). The equilibrium mixture contains 3-12% of PAA, 1-8% of AA, and 10-35% of H2O2. The molar ratio of hydrogen peroxide to acetic acid in the equilibrium mixture is within the range 4-30. The reaction solution additionally contains sulfuric acid 10-25%. There prevails a reduced pressure in the reactor, and the equilibrium mixture is heated so that the peracetic acid and the water vaporize and leave the reaction mixture. The vapor phase is directed to a distillation column, where peracetic acid concentrates relative to the other components. The peracetic acid vapor is directed from the distillation column to a condenser, where the peracetic acid condenses to an aqueous solution having a PAA concentration of 20-60%. A portion of the solution is removed into the product container and a portion is returned to the distillation column.
U.S. Pat. No. 4,904,821 discloses a process for the production of peracetic acid so that the peracetic acid is in an organic solvent. In the production process the molar ratio of hydrogen peroxide to acetic acid is 1-2. The hydrogen peroxide is fed into the reactor as a 30-35% aqueous solution so that the weight ratio of hydrogen peroxide to water is 0.42-0.54. The amount of sulfuric acid is 20-30% of the weight of the entire equilibrium mixture. The temperature is within the range 55-70xc2x0 C. and the pressure is 100-200 mbar. The vapor phase formed in the reaction, containing peracetic acid, acetic acid and water, is directed to the absorption zone into contact with an organic phosphate circulating in the opposite direction, whereupon a portion of the peracetic acid and acetic acid passes into the organic solvent.
The sulfuric acid amounts presented in the above-mentioned patent publications are highly typical of the production technology currently used. The acid catalyst speeds up the reaction between carboxylic acid and hydrogen peroxide and the formation of an equilibrium mixture.
Patent publication GB 1 014 361 discloses results of tests aimed at determining how much sulfuric acid is required in the reaction mixture. It is evident from the results presented in the table of the patent publication that, if sulfuric acid is used in an amount of 1%, the PAA concentration in the distillate remains low and the retention time of the reaction is more than three times that in tests in which sulfuric acid was used in an amount of 9-20%. On the other hand, if sulfuric acid is used in an amount higher than 20%, it is necessary to add water to the reaction mixture. In this case the sulfuric acid and water take up too large a proportion of the reactor volume, and the amount of product per unit volume of the reactor decreases. On the basis of these tests, the patent publication presents a process for the production of peracetic acid wherein hydrogen peroxide and acetic acid, the molar ratio of which is 0.3-5, are allowed to react in a reaction mixture containing sulfuric acid 5-20% and water 20-75%, at a temperature of 20-80xc2x0 C. and a pressure of 15-350 mmHg. The reaction product vaporizing out from the mixture, the product containing PAA, AA and water, is directed to a condenser and is recovered.
In the production of distilled percarboxylic acid the reaction medium contains large amounts of sulfuric acid and metallic impurities. Impurities accumulate in the reaction medium, and therefore the medium is removed at predetermined times as a bottom product when the amount of impurities grows too large. In conventional production processes this is an unusable waste, and so attempts have been made to maintain the amount of the solution to be removed as small as possible. The metallic impurities are derived mainly from the carboxylic acid and the hydrogen peroxide. Metallic impurities are detrimental, since, among other things, they decompose hydrogen peroxide.
Conventionally the bottom product has thus been a waste unusable owing to the large amounts of sulfuric acid and impurities present in it. In the conventional process there is additionally always the risk that so large amounts of metallic impurities accumulate in the bottom product that they tend to decompose the percarboxylic acid solution, and a dangerously rapid increase in pressure may result.
Thus there is in the field a clear need for a production process wherein, on the one hand, the bottom product could be used as such and, on the other hand, the production process would be safe. In this case the improving of the usability of the bottom product would at the same time improve the profitability of the entire production process.